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Thread: What is subnet Masking?

  1. #1
    Join Date
    Jul 2009
    Posts
    96

    What is subnet Masking?

    Hello,

    I'm trying to learn some tools to see what happens under the hood of my computer. I'm trying to learn nmap. I'm reading a book. I could not understand what is the subnet mask. I understand that computers communicate through IP addresses, but what is the purpose of subnet Masking and how it works?

  2. #2
    Join Date
    Dec 2007
    Posts
    112

    Re: What is subnet Masking?

    A mask used to determine what subnet an IP address belongs to. An IP address has two components, the network address and the host address. For example, consider the IP address 150.215.017.009. Assuming this is part of a Class B network, the first two numbers (150.215) represent the Class B network address, and the second two numbers (017.009) identify a particular host on this network.

    Subnetting enables the network administrator to further divide the host part of the address into two or more subnets. In this case, a part of the host address is reserved to identify the particular subnet. This is easier to see if we show the IP address in binary format. The full address is:

    10010110.11010111.00010001.00001001

    The Class B network part is:

    10010110.11010111

    and the host address is

    00010001.00001001

    If this network is divided into 14 subnets, however, then the first 4 bits of the host address (0001) are reserved for identifying the subnet.

    The subnet mask is the network address plus the bits reserved for identifying the subnetwork. (By convention, the bits for the network address are all set to 1, though it would also work if the bits were set exactly as in the network address.) In this case, therefore, the subnet mask would be 11111111.11111111.11110000.00000000. It's called a mask because it can be used to identify the subnet to which an IP address belongs by performing a bitwise AND operation on the mask and the IP address. The result is the subnetwork address:

    Subnet Mask 255.255.240.000 11111111.11111111.11110000.00000000
    IP Address 150.215.017.009 10010110.11010111.00010001.00001001
    Subnet Address 150.215.016.000 10010110.11010111.00010000.00000000

    The subnet address, therefore, is 150.215.016.000.

    Source : webopedia.com

  3. #3
    Join Date
    Dec 2007
    Posts
    202

    Re: What is subnet Masking?

    Subnet: A portion of a network that shares a common address component. In TCP / IP subnets are defined as all devices whose IP addresses have the same prefix. For example, all devices with IP addresses that begin with 100.100.100 would be part of the same subnet. Dividing a network into subnets is useful for both security and performance reasons. IP networks are divided using a subnet mask.

    A Subnet mask used to determine what subnet an IP address belongs a. An IP address has two components of the network address and host address. For example, consider the IP address 150.215.017.009. Assuming that this is part of a class B network the first two numbers (150.215) represent the network address class B and the second two numbers (017.009) identifies a particular host on this network.

  4. #4
    Join Date
    Dec 2007
    Posts
    195

    Re: What is subnet Masking?

    In simple terms its a mechanism to split your network into small or more manageable networks hence the term "subnet". It is used to determine what the network address is for any given IP address, you merely have to convert both octal addresses into binary, and do a bitwise and operation.

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